Method for manufacturing stamper for information medium manufacture, stamper, and photoresist master disk

ABSTRACT

A stamper with a sharp uneven pattern is obtained, and high precision information media can be manufactured using this stamper.  
     A photoresist master  100  is manufactured by forming a light absorption layer  103  with a film thickness T that satisfies T&gt;180 (nm) and a photoresist layer  104 , in that order, on top of a substrate  102 , and then forming an uneven pattern  106  in the photoresist layer  104  by forming and developing a latent image, and a stamper  120  is manufactured by forming a Ni thin film  108  on top of the uneven pattern  106  of the photoresist master  100  by electroless plating, forming a Ni film  110  on top of this Ni thin film  108  by electroforming, and then separating the Ni thin film  108  and the Ni film  110  from the photoresist master  100.

TECHNICAL FIELD

The present invention relates to a stamper used during the manufactureof an information medium such as an optical disc or a magnetic disccomprising an uneven pattern such as grooves and prepits, a method ofmanufacturing the stamper using a photoresist master, a photoresistmaster, and an information medium manufactured using the stamper.

BACKGROUND ART

Optical discs, which represent one type of information media, arecurrently available in two different varieties: optical recording discswhich enable writing or rewriting of information, and read-only discs inwhich the information has been pre-recorded onto the disc.

A groove (guide channel) that is used for tracking and the like isformed in the disc substrate of an optical recording disc, and arecording layer comprising a phase change material or an organic dyematerial is laminated on top of the disc substrate. When the laser beamis irradiated onto the recording layer, the recording layer undergoes achemical or physical change, thus forming a recording mark. In contrast,in the case of a read-only disc, recording marks (information pits) areformed in advance as part of an uneven pattern on the disc substrate.When a reading laser beam is irradiated onto these recording marks, thequantity of reflected light varies, and by detecting these variations,the information is able to be read (played back).

In order to manufacture a disc substrate with an uneven pattern ofgrooves, information pits, and the like, a stamper is used in which thenegative pattern (which is itself a type of uneven pattern) of thedesired uneven pattern has been formed. For example, a method ofmanufacturing a disc substrate by conducting injection molding using amold with the above stamper secured inside the cavity, therebytransferring the negative pattern to the resin used to fill the cavity,is common.

A stamper with an uneven pattern is usually formed from a metal stampercontaining Ni or the like. In the steps required for manufacturing thisstamper, first a photoresist master with the negative pattern of theuneven pattern of the stamper is prepared, and a metal film is thenformed on this photoresist master by plating. Subsequently, the metalfilm is separated from the photoresist master, and then subjected to aseries of predetermined treatments such as surface washing to form thestamper.

As follows is a description of the manufacturing process for aphotoresist master 1, with reference to the conventional photoresistmaster 1 shown in FIG. 7. First, a photoresist layer 4 is formed on topof a glass substrate 2. Next, the photoresist layer 4 is exposed using apatterning laser beam such as a laser, and the latent image pattern isdeveloped. This enables the production of the photoresist master 1 withan uneven pattern 6 formed in the photoresist layer 4.

In order to use this photoresist master 1 to prepare a stamper 20 byplating, first, as shown in FIG. 8, a thin metal film 8 containing a Nimaterial or the like is formed on the surface of the uneven pattern 6using a process such as electroless plating, thereby impartingconductivity to the photoresist master 1.

Subsequently, electroplating is conducted with the thin metal film 8 asa backing, thereby forming a metal film 10 containing Ni or the like. Byremoving the thin metal film 8 and the metal film 10 from thephotoresist master 1, a stamper 20 containing the transferred unevenpattern 6 can be obtained.

In recent years, as the capacity of optical recording media hasincreased, uneven patterns such as grooves have become much finer,meaning errors in the pattern shape have a large effect on the recordingand reading accuracy. Accordingly, it is desirable to form a sharpuneven pattern on the disc substrate, but in order to achieve this sharppattern, the uneven pattern of the photoresist layer 4, which is thebasis for the pattern, must be formed with a high level of precision(sharpness).

The minimum width of the latent image pattern formed on the photoresistlayer 4 is limited by the spot diameter of the laser beam when itreaches the photoresist layer 4. When λ is the laser wavelength, and NAis the numerical aperture of the objective lens of the irradiatingoptical system, then the spot diameter w is represented by the formulaw=k·λ/NA. k is a constant that is determined by the aperture shape ofthe objective lens and the intensity distribution of the incident lightbeam.

However, even in the case of patterns with widths that theoretically donot exceed the spot diameter limit, if the photoresist layer 4 is thin,then problems of inadequate sharpness can arise due to factors such asshallowness of the uneven pattern transferred to the stamper, orrounding of the shape of the uneven pattern (this is known as patternsag). It is thought that these problems are caused by fluctuationsoccurring in the thickness of the photoresist layer 4 (this is known asfilm thinning) during typical exposure and developing operations. It isthought that these thickness fluctuations are caused by laser beamreflection between the photoresist layer 4 and the glass substrate 2,with this reflection causing excessive exposure of the photoresist layer4.

The inventor of the present invention has clarified that forming a lightabsorption layer between the glass substrate 2 and the photoresist layer4 is an effective way of resolving these problems. By so doing, thelight absorption layer can absorb the laser beam and suppress any lightreflection, and consequently a sharper exposure and development can beachieved than in a conventional process.

However, based on further research, the inventor of the presentinvention noticed that a photoresist master 1 with a light absorptionlayer displayed some problems relating to the formation of the thinmetal film 8 by electroless plating. Specifically, it was surmised thata photoresist master 1 in which the light absorption layer was partiallyexposed was prone to increases in fine irregularities (fine defects)during the electroless plating process. In other words, it wasdiscovered that even though the same method was used to form the thinmetal film, on some occasions when the stamper was removed, for somereason or other fine irregularities (fine defects) had been formed onthe surface of the uneven pattern of the stamper. During playback thesefine irregularities manifest as noise, meaning that despite the attemptto improve the recording capacity by effectively utilizing a lightabsorption layer, in reality a decrease occurs in the recording andplayback performance.

If this problem can be resolved, then the manufacture of a stamper witha sharp uneven pattern should be possible using a photoresist masterwith a light absorption layer. In other words, it became clear that asharp uneven pattern that had been formed on the master through theeffects of the light absorption layer, could be transferred faithfullyto a stamper.

DISCLOSURE OF THE INVENTION

The present invention is directed to a solution to the above describeddisadvantage, and it is an object of the present invention to provide amethod of manufacturing a stamper in which shape errors are suppressedduring the electroless plating process, as well as a stampermanufactured using such a method.

As a result of intensive research on methods of manufacturinginformation media such as optical discs and magnetic discs (discretemedia), the inventor of the present invention discovered a method offorming a sharp uneven pattern on a stamper. In other words, the aboveobject can be achieved by the present invention described below.

(1) A method of manufacturing a stamper for manufacturing an informationmedium, comprising the steps of: manufacturing a photoresist master byforming a light absorption layer with a film thickness T that satisfiesT>180 (nm) and a photoresist layer, in that order, on top of asubstrate, irradiating light onto said photoresist layer to form alatent image from an opposite surface to that which contacts said lightabsorption layer, and then developing said latent image to form anuneven pattern; forming a thin metal film on top of said uneven patternof said photoresist master; forming a metal film on top of said thinmetal film; and forming a stamper by separating said thin metal film andsaid metal film from said photoresist master.

(2) The method of manufacturing a stamper according to (1), wherein thefilm thickness T of said light absorption layer satisfies T>200 (nm).

(3) A stamper for manufacturing an information medium, in a surface ofthe stamper an uneven pattern being formed in advance, the stamper beingmanufactured by the steps of: manufacturing a photoresist master byforming a light absorption layer with a film thickness T that satisfiesT>180 (nm) and a photoresist layer, in that order, on top of asubstrate, irradiating light onto said photoresist layer to form alatent image from an opposite surface to that which contacts said lightabsorption layer, and then developing said latent image to form anuneven pattern; forming a thin metal film on top of said uneven patternof said photoresist master; forming a metal film on top of said thinmetal film; and forming the stamper by separating said thin metal filmand said metal film from said photoresist master.

(4) A stamper according to (3), wherein the film thickness T of saidlight absorption layer satisfies T>200 (nm).

(5) A photoresist master comprising a substrate, a light absorptionlayer laminated on top of said substrate, and a photoresist layer whichis laminated on top of said light absorption layer and is capable ofhaving an uneven pattern formed therein by forming and subsequentlydeveloping of a latent image, wherein a film thickness T of said lightabsorption layer satisfies T>180 (nm) and preferably T>200 (nm).

(6) An information medium, in which a final uneven pattern is formed byusing, as a negative pattern, an uneven pattern of a stampermanufactured by the steps of: manufacturing a photoresist master byforming a light absorption layer with a film thickness T that satisfiesT>180 (nm) and a photoresist layer, in that order, on top of asubstrate, irradiating light onto said photoresist layer to form alatent image from an opposite surface to that which contacts said lightabsorption layer, and then developing said latent image to form anuneven pattern; forming a thin metal film on top of said uneven patternof said photoresist master; forming a metal film on top of said thinmetal film; and forming the stamper by separating said thin metal filmand said metal film from said photoresist master.

(7) The information medium according to (6), wherein said final unevenpattern is formed by direct transfer of said uneven pattern from saidstamper.

(8) The information medium according to (6), wherein said final unevenpattern is formed by transfer of an uneven pattern from a motherstamper, and said uneven pattern of said mother stamper is formed bytransfer of said uneven pattern using said stamper as a master stamper.

(9) The information medium according to (6), wherein said final unevenpattern is formed by transfer of an uneven pattern from a child stamper,and said uneven pattern of said child stamper is formed by transfer ofan uneven pattern from a mother stamper, which has been formed bytransfer of said uneven pattern using said stamper as a master stamper.

The inventor of the present invention applied a metal catalyst to aphotoresist master comprising a light absorption layer, and discoveredthat the synergistic effect of the advantages offered by the lightabsorption layer and the provision of the metal catalyst enabled theformation of an uneven pattern with better sharpness than thatobtainable by conventional processes. In addition, noticing the factthat a photoresist master in which the light absorption layer ispartially exposed can be prone to increases in fine irregularities (finedefects) during the electroless plating process, the inventor of thepresent invention investigated in further detail, and discovered that byensuring that the film thickness T of the light absorption layersatisfies T>180 (nm), the quantity of fine defects on the surface of theuneven pattern of the stamper can be reduced.

The reason for this observation is thought to be as follows, althoughthis is only conjecture.

Following application of the metal catalyst, during removal of Sn usingan accelerator, the accelerator penetrates into the partially exposedlight absorption layer, reaching as far as the glass substrate surface.At this point, the coupling agent layer that is normally applied to thesurface of the glass substrate undergoes some form of reaction with theaccelerator, generating a gas and causing the formation of fineirregularities. In the present invention, the thickness of the lightabsorption layer is thickened to at least 180 nm to ensure that theaccelerator cannot reach as far as the glass substrate surface, andconsequently fine irregularities do not develop. As a result, thesynergistic effect of this thicker layer, in combination with the factthat a sharp uneven pattern is formed due to the advantages offered bythe light absorption layer, enables the formation of a stamper in which,compared with conventional processes, an even sharper uneven pattern hasbeen transferred faithfully to the stamper.

As a result, the grooves, information pits, and the like of aninformation medium can also be formed with good sharpness, and thismakes it possible to improve the recording and playback characteristics.Furthermore, because the invention is compatible with future ongoingminiaturization of uneven patterns, it also enables increases in theinformation memory (recording) capacity of information media.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a photoresist master accordingto an embodiment of the present invention;

FIG. 2(A) is a cross-sectional view showing a state during themanufacture of a stamper using the same photoresist master;

FIG. 2(B) is a cross-sectional view showing the manufactured stamper;

FIG. 3(A) is a diagram showing the result of an AFM analysis of theuneven pattern formed on a stamper according to an example of thepresent invention;

FIG. 3(B) is a line diagram showing the cross-sectional shape of theuneven pattern determined on the basis of the AFM analysis;

FIG. 4(A) is a diagram showing the result of an AFM analysis of theuneven pattern formed on a stamper according to a comparative example ofthe present invention;

FIG. 4(B) is a line diagram showing the cross-sectional shape of theuneven pattern determined on the basis of the AFM analysis;

FIG. 5 is a line diagram showing the uneven state of the stamper surfaceof the above example, as measured by a scanning electron microscope;

FIG. 6 is a line diagram showing the uneven state of the stamper surfaceof the above comparative example, as measured by a scanning electronmicroscope;

FIG. 7 is a cross-sectional view showing a conventional photoresistmaster; and

FIG. 8 is a cross-sectional view showing a state during the manufactureof a stamper using the same conventional photoresist master.

BEST MODE FOR CARRYING OUT THE INVENTION

As follows is a detailed description of an embodiment of the presentinvention, based on the drawings.

FIG. 1 shows a photoresist master 100 according to the sample embodimentof the present invention. This photoresist master 100 comprises a glasssubstrate 102, a light absorption layer 103 with a film thickness T thatsatisfies T>180 (nm), and preferably T>200 (nm) laminated on top of thisglass substrate 102, and a photoresist layer 104 laminated on top ofthis light absorption layer 103. A latent image of an uneven pattern isformed on the photoresist layer 104, on the opposite side from the lightabsorption layer 103 (the top side in FIG. 1), by exposure with apatterning laser beam, and development of this latent image causes theremoval of a portion of the photoresist layer, forming an uneven pattern106. Following developing, portions of the light absorption layer 103are exposed at the bottom surfaces of the concave sections of the unevenpattern 106.

The numeral 107 in FIG. 1 shows a non-uneven area in which the unevenpattern has not been formed.

As described below, the aforementioned uneven pattern 106 becomes thepattern surface 206 of a stamper 120. Furthermore, the area in which theuneven pattern has not been formed becomes the mirror surface 207 of thestamper 120.

During exposure, the patterning laser beam is absorbed by the lightabsorption layer 103, thereby suppressing light reflection and enablingthe formation of fine unevennesses with good sharpness.

Pd (106A) is then provided on the surface of the uneven pattern 106 ofthe photoresist master 100. Specifically, a catalyst (a Pd-Sn compound)is adsorbed onto the surface of the uneven pattern 106, and anaccelerator is used to remove only the Sn from the catalyst, leaving thePd deposited on the surface of the uneven pattern 106. The surface ofthe uneven pattern 106 on which the Pd has been provided may then bewashed with a liquid. For example, by washing the surface of the unevenpattern 106 using water (and preferably ultra pure water), thegeneration of fine irregularities can be even better suppressed.

In FIG. 1, the Pd (106A) is shown schematically as a thin film, but thisdoes not represent the actual state of the Pd.

FIG. 2(A) shows the stamper 120 formed using the above photoresistmaster 100.

In this formation process, first electroless plating is used to form aNi thin film 108 on the surface of the uneven pattern 106 on which thePd has been deposited.

During this formation, a reducing agent in the plating solution isoxidized at the Pd surface, which possesses catalytic activity, andemits an electron, and this electron reduces a Ni ion in the solution,enabling the Ni thin film 108 to effectively conform to, and follow theuneven pattern 106.

Subsequently, a current is passed through the surface, using the Ni thinfilm 108 as a backing, and electroplating is used to form a Ni film 110.If the Ni thin film 108 and the Ni film 110 are then removed from thephotoresist master 100, then as shown in FIG. 2(B), a stamper 120comprising an accurately transferred uneven pattern 106 can be obtained.At this point, the aforementioned Pd (106A) remains bonded to the Nithin film 108.

In this stamper 120, the pattern surface 206 is formed in the areacorresponding with the uneven pattern 106, and the mirror surface 207 isformed in the area corresponding with the non-uneven area 107.

Although not specifically shown in the drawings, the stamper 120 canthen be installed in a mold, and injection molding or the like is usedto manufacture an optical disc substrate having a final uneven patterncreated by transferring the uneven pattern as a negative pattern. Inaddition to using the stamper 120 to manufacture optical discsubstrates, the stamper 120 can also be used as a master stamper forpreparing a mother stamper by an electroforming process, and this motherstamper can then be used to manufacture optical discs.

In addition, this mother stamper could also be used as a master forpreparing a child stamper, and this child stamper can then be used tomanufacture the optical discs. In other words, the stamper 120 of thepresent invention need not necessarily be used directly for themanufacture of optical discs, but may also be used indirectly for suchoptical disc manufacture, as the master stamper used in the preparationof a mother stamper or the like.

In the photoresist layer 104 of this embodiment, the provision of thelight absorption layer 103 enables a well defined latent image to beprojected, thus enabling a sharp uneven pattern 106 to be produced. As aresult, sagging of the uneven pattern formed on the stamper 120 issuppressed. In addition, by ensuring that the film thickness T of thelight absorption layer 103 satisfies T>180 (nm), and preferably T>200(nm), the number of fine irregularities (fine defects) on the surface ofthe uneven pattern transferred to the stamper 120 can be reducedmarkedly. Accordingly, the sharp uneven pattern 106 can be transferredto the stamper 120 with good retention of this sharpness, and by usingthis stamper 120, optical recording media with suppressed noise levelsand good levels of recording and reading (playback) accuracy can beproduced.

Furthermore, even in those cases where exposure is stopped prior toexposure of the light absorption layer, that is, partway through thethickness of the photoresist layer, a synergistic effect is stillobtained due to the applied Pd and the light absorption layer, andconsequently the sharp uneven pattern is able to be transferred to thestamper with good retention of this sharpness, in a similar manner tothat described above.

In addition, in the present embodiment only the case involving a Niplating treatment was described, but the present invention is notlimited to this case, and other metal plating can also be used.

Furthermore, the stamper described above is applicable not only tooptical discs, but can also be applied generally to the manufacture ofinformation media, including magnetic discs (discrete media).

EXAMPLES Example: Stamper No. 1

Following formation of a layer of a coupling agent on top of a polishedglass substrate, a light absorption layer was formed by spin coating.The application liquid used was SWK-T5D60 (manufactured by Tokyo OhkaKogyo Co., Ltd.) containing 4,4′-bis(diethylamino)benzophenone as alight absorption agent. The applied layer was baked at 200° C. for 15minutes to cure the layer and remove residual solvent, thus forming alight absorption layer of 200 nm in thickness. Subsequently, aphotoresist (DVR100, manufactured by Zeon Corporation) was spin coatedonto the light absorption layer, and residual solvent was vaporized bybaking, thus forming a photoresist layer of 25 nm in thickness.

Subsequently, using a cutting machine manufactured by Sony Corporation,and targeting the formation of a groove pattern with a track pitch of320 nm and a groove width of 150 nm, the photoresist layer was exposedwith a Kr laser (wavelength=351 nm) and subsequently developed to forman uneven pattern, thus producing a photoresist master.

Following activation of the surface of the photoresist layer of thisphotoresist master using a surfactant, a catalyst (a Pd, Sn colloid) wasapplied as a preliminary treatment to electroless plating. Anaccelerator (HBF₄ solution) was then used to remove the Sn and achievedeposition of the Pd onto the surface of the photoresist master, thuscompleting the preparation for electroless plating.

The photoresist master was then immersed in a NiCl₂ solution, and a Nithin film was formed by electroless plating. Electroplating was thenconducted with this Ni thin film as a backing, thus forming a Ni film.The laminate formed from this Ni thin film and the Ni film was separatedfrom the master, the rear surface was polished, and the surface waswashed, thus completing production of a stamper No. 1.

Comparative Example: Stamper No. 2

With the exception of altering the film thickness of the lightabsorption layer to 140 nm, a stamper No. 2 was prepared in the samemanner as the preparation of the stamper No. 1.

Comparative Example: Stamper No. 3

With the exception of not providing a light absorption layer, a stamperNo. 3 was prepared in the same manner as the preparation of the stamperNo. 1.

(Evaluation Results 1)

The shape of the uneven pattern formed on each stamper was confirmed byinspection using an AFM (atomic force microscope). A silicon nitride(SiN) probe tip was used for the AFM probe. Measurement was conductedusing a non-contact mode, and the variations in atomic force between thesample and the probe were converted to an image.

FIG. 3(A) shows the AFM image of the stamper No. 1, and FIG. 3(B) is aline diagram showing the cross-sectional shape of the same image.Similarly, FIG. 4(A) shows the AFM image of the stamper No. 3, and FIG.4(B) is a line diagram showing the cross-sectional shape of the sameimage. In the AFM images, the dark areas of high dot density representthe concave sections within the uneven patterns, and the areas of lowdot density or the white areas represent the convex sections, and theseconcave and convex sections correspond with the convex and concavesections respectively of the uneven pattern on the photoresist master.In FIG. 3(B) and FIG. 4(B), the uneven patterns are formed with a pitchof 0.32 μm.

As is evident from comparing FIG. 3 and FIG. 4, in the stamper No. 1that was manufactured in accordance with the present invention, theeffect of the light absorption layer resulted in the formation of asharp pattern, and the effect of using a film thickness T for the lightabsorption layer of T>180 (nm) enabled the pattern to be transferredfaithfully.

(Evaluation Results 2)

The uneven states of the stamper No. 1 and the stamper No. 2, asmeasured by a scanning electron microscope (10,000× magnification), areshown in FIG. 5 and FIG. 6 respectively. By comparing FIG. 5 and FIG. 6it is evident that whereas no fine irregularities can be seen for thestamper No. 1, in the stamper No. 2, fine irregularities that appear asindentations with a width of approximately 1 μm are clearly visible atapproximately 3 μm and 8.5 μm along the horizontal axis. In FIG. 5 andFIG. 6, the unevennesses that appear with a pitch of approximately 0.3μm represent the uneven pattern to be formed in the present invention.

INDUSTRIAL APPLICABILITY

In the present invention, the light absorption layer contacting thephotoresist layer enables the formation of a sharp uneven pattern on thephotoresist master, and adjustment of the film thickness of the lightabsorption layer enables the production of a stamper that is faithful tothis uneven pattern.

1. A method of manufacturing a stamper for manufacturing an informationmedium, comprising the steps of: manufacturing a photoresist master byforming a light absorption layer with a film thickness T that satisfiesT>180 (nm) and a photoresist layer, in that order, on top of asubstrate, irradiating light onto said photoresist layer to form alatent image from an opposite surface to that which contacts said lightabsorption layer, and then developing said latent image to form anuneven pattern; forming a thin metal film on top of said uneven patternof said photoresist master; forming a metal film on top of said thinmetal film; and forming a stamper by separating said thin metal film andsaid metal film from said photoresist master.
 2. The method ofmanufacturing a stamper according to claim 1, wherein the film thicknessT of said light absorption layer satisfies T>200 (nm).
 3. A stamper formanufacturing an information medium, in a surface of the stamper anuneven pattern being formed in advance, the stamper being manufacturedby the steps of: manufacturing a photoresist master by forming a lightabsorption layer with a film thickness T that satisfies T>180 (nm) and aphotoresist layer, in that order, on top of a substrate, irradiatinglight onto said photoresist layer to form a latent image from anopposite surface to that which contacts said light absorption layer, andthen developing said latent image to form an uneven pattern; forming athin metal film on top of said uneven pattern of said photoresistmaster; forming a metal film on top of said thin metal film; and formingthe stamper by separating said thin metal film and said metal film fromsaid photoresist master.
 4. A stamper according to claim 3, wherein thefilm thickness T of said light absorption layer satisfies T>200 (nm). 5.A photoresist master comprising a substrate, a light absorption layerlaminated on top of said substrate, and a photoresist layer which islaminated on top of said light absorption layer and is capable of havingan uneven pattern formed therein by forming and subsequently developingof a latent image, wherein a film thickness T of said light absorptionlayer satisfies T>180 (nm) and preferably T>200 (nm).
 6. An informationmedium, in which a final uneven pattern is formed by using, as anegative pattern, an uneven pattern of a stamper manufactured by thesteps of: manufacturing a photoresist master by forming a lightabsorption layer with a film thickness T that satisfies T>180 (nm) and aphotoresist layer, in that order, on top of a substrate, irradiatinglight onto said photoresist layer to form a latent image from anopposite surface to that which contacts said light absorption layer, andthen developing said latent image to form an uneven pattern; forming athin metal film on top of said uneven pattern of said photoresistmaster; forming a metal film on top of said thin metal film; and formingthe stamper by separating said thin metal film and said metal film fromsaid photoresist master.
 7. The information medium according to claim 6,wherein said final uneven pattern is formed by direct transfer of saiduneven pattern from said stamper.
 8. The information medium according toclaim 6, wherein said final uneven pattern is formed by transfer of anuneven pattern from a mother stamper, and said uneven pattern of saidmother stamper is formed by transfer of said uneven pattern using saidstamper as a master stamper.
 9. The information medium according toclaim 6, wherein said final uneven pattern is formed by transfer of anuneven pattern from a child stamper, and said uneven pattern of saidchild stamper is formed by transfer of an uneven pattern from a motherstamper, which has been formed by transfer of said uneven pattern usingsaid stamper as a master stamper.